When a single dynamic range compressor (DRC) is applied to the entire bandwidth of an audio signal, the resulting gain reduction is applied to the entire signal (i.e., across its entire spectrum). The effect is similar to turning down a volume control, but automatically and in response to an increase in the signal's level, e.g., in order to attempt to avoid playback distortion and/or equipment damage, or simply to avoid excessive volume that might disturb others. Unfortunately, one problem arising from this simplistic approach to range compression is that an individual loud event, e.g. originating from a single instrument, such as a kick drum, can trigger gain reduction for the entire audio signal, even though other components of the audio do not need, or are not desired, to be reduced in volume in order to achieve the desired effect. For example, as a result of such whole-signal processing, the vocals within the audio signal might be reduced to an unacceptably low level during the period that gain reduction is being applied.
This problem conventionally has been mitigated by instead using a multiband dynamic range compressor, which employs different compressors for different frequency bands. With this approach, a loud event in one frequency band triggers gain reduction in that band only, leaving the other bands largely untouched. For example, a loud event from a kick drum will be compressed solely by a compressor covering only the particular low-frequency band in which the corresponding burst of signal energy is occurring, leaving alone sounds that occur in other (e.g., mid-range and high-frequency bands).
As shown in FIG. 1, a conventional multiband compressor 10 comprises a set 12 of filters (including individual filters 12A-C) that splits the audio signal into two or more frequency bands (three in the present example). Each frequency band is then processed by its own separate compressor (with the output of each of filters 12A-C coupled to one of the compressors 14A-C, respectively). The outputs from these compressors 14A-C are recombined in adder 15 to form the output signal 18.
Unfortunately, one significant problem with multiband compressors such as compressor 10 is that the compressed signal 18 often overshoots above the target compression threshold. This situation primarily occurs as a result of input audio signal energy within the crossover or overlapping regions of the frequency bands. In this regard, each of the filters 12A-C outputs a corresponding frequency band 22A-C, as illustrated in FIG. 2, with adjacent frequency bands overlapping (e.g., overlap region 24A between bands 22A&B and overlap region 24B between bands 22B&C).
The overshooting effect noted in the preceding paragraph is illustrated in FIG. 3 for a two-band dynamic range compressor in which a compressor is applied to the passband, and the stopband is left uncompressed. For ease of reference, the term “uncompressed”, as used herein without further elaboration, means that the subject signal (typically, a particular band-limited signal or, more succinctly, frequency band) has not been range-compressed. In this example, the passband crossover frequencies are set at 1 kilohertz (kHz) and 10 kHz. Its compressor has a 20 decibel (dB) maximum gain and a desired limiting threshold of 0 dB. The 20-dB gain can be clearly seen when the input level is low (i.e., the lower curves, such as curve 30 and below, shown in FIG. 3). When the input level increases (represented by the higher curves, such as curve 31 and above), the limiter starts to kick in, to limit the output level, ideally to a maximum of 0 dB, which is the desired limiting threshold 28 in the current example. However, this limit 28 typically is only imposed around the center frequency 32 (which is 3.162 kHz in the current example) and for frequencies far away from the crossover frequencies 34. As can be seen in FIG. 3, within and around the crossover frequencies 34, the input signal energy is not fully suppressed by the stopband filter, causing the output signal to overshoot this desired limit 28, so that it reaches levels as high as almost 6 dB.
Conventional approaches to mitigating this problem include: (1) E. Lindemann, “The continuous frequency dynamic range compressor,” Proceedings of 1997 Workshop on Applications of Signal Processing to Audio and Acoustics, New Paltz, N.Y., USA, 1997, DOI: 10.1109/ASPAA.1997.625580, ISBN: 0-7803-3908-8, which addresses this problem by increasing the number of bands and extending the overlap region between bands; (2) E. Vickers, “The Non-Flat and Continually Changing Frequency Response of Multiband Compressors”. 2010, Audio Engineering Society (AES) Convention 129, which employs a frequency-domain approach that examines the input signal near the crossover regions and recalculates the band boundaries so that the peak falls entirely within a single band (thereby limiting the amount of signal energy that occurs in the crossover regions); and (3) D. Traore, J. Atkins, “Compensation of Crossover Region Overshoot in Multiband Compression”, 2014 AES Convention 136, in which a frequency-decomposition module (which attempts to provide an estimate of the overshoot by determining the amount of input signal energy within the crossover regions) and a gain-compensation filter (which attempts to limit the effects of such anticipated overshoot) are provided at the beginning of the gain path of each compressor.